Active Isolator Circuit With Parallel Amplifiers for Low-Voltage RF Linearity
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Solution Overview
Problem
Current active isolators for RF circuits face challenges with impedance mismatch, leading to in-band amplitude and phase ripple, reduced gain, and high power consumption, especially at low power supply voltages, which degrades linearity and increases power loss.
Innovation Solution
An active isolator circuit design utilizing a common gate amplifier and a common drain amplifier in parallel configuration, with an RF signal blocking device, such as a degenerative inductor, to suppress RF signals and function as a current source biasing the common drain amplifier, operating at low power supplies like 1V or less.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If passive components are used in matching networks for impedance transformation, then impedance matching is achieved, but the components become more lossy and larger in size
Solution Approach 1:
The patent replaces passive mechanical/electrical components (matching networks with inductors and capacitors) with an active electronic system using transistors configured as amplifiers. The common gate and common drain amplifiers actively transform impedance through their transistor characteristics rather than passive component values, reducing power loss and improving efficiency while maintaining impedance matching performance.
2Reliability
If higher supply voltage is used to maintain transistors in desired operating conditions, then linearity is improved, but power consumption increases
Solution Approach 1:
The patent changes the operating parameters of the transistor circuit by using a low supply voltage (1V or less) combined with specific biasing configurations. The common gate amplifier is biased with a first DC voltage and the common drain amplifier with a second DC voltage, allowing the transistors to operate in their linear region while consuming minimal power. This parameter optimization achieves both linearity and low power consumption simultaneously.
3Area of stationary object
If smaller transistors are used to reduce circuit size, then integration is improved, but breakdown voltages decrease requiring lower operating voltages
Solution Approach 1:
The patent makes the transistor circuit universally adaptable to low voltage operation by designing it to function effectively at 1V or less. The parallel configuration of common gate and common drain amplifiers with proper biasing allows the circuit to maintain its isolator function, impedance transformation capability, and linearity while operating at the lowest practical voltage level, thus accommodating smaller transistors without sacrificing reliability.
4Area of stationary object
If passive elements are used in current isolator topologies, then circuit size is reduced, but bandwidth performance deteriorates and power loss increases
Solution Approach 1:
The patent replaces passive elements (inductors, capacitors, transformers) with an active transistor-based system. The common gate and common drain amplifiers use the transistor's inherent amplification and impedance transformation properties to achieve wideband performance. This active approach eliminates the bandwidth limitations and power losses associated with passive components while maintaining compact circuit size suitable for integration.
Data Source
AI summary
A wideband low power active isolator that may operate with a low voltage supply and provide improved linearity and insertion loss is described. The active isolator includes parallel connected common gate amplifier and common drain amplifier that are implemented using active transistors. A RF choke configured to suppress RF signal is coupled between input to the common gate amplifier and the ground such that the common gate amplifier also functions as a current source biasing the common drain amplifier.